Hitherto, a bumper attached to the body of an automobile or the like is an example of a bending member on which a bending load due to external force acts, and is made of a thin material. Such a bumper is formed by shaping, for example, a steel sheet of 980 MPa class.
The main role of a bumper is to deform to absorb energy at the time of collision, to transmit the shock load to the left and right side members, and to cause the side members to absorb the energy at the time of collision by causing the side members to deform. That is to say, by causing the side members to deform and absorb energy, the deformation of the cabin of the automobile is prevented as designed, and the occupants are protected from shock.
Patent Literature 1 discloses an automotive bumper reinforcement in which the thickness of the part on the compression flange side of the bending neutral axis of webs forming a hollow rectangular section is larger than the thickness of the part on the tension flange side. Patent Literature 2 discloses an automotive bumper apparatus in which, of three ribs of a bumper reinforcement, an intermediate rib has a thickness larger than that of the other ribs, and the decline in energy absorption capability when the three ribs are buckled is thereby prevented. Patent Literature 3 discloses integrally forming a bumper having a two-part closed section by folding a single sheet.
Patent Literature 4 discloses a bending strength member in which an FRP (Fiber Reinforced Plastics) material is provided on the flange surface on the opposite side from the flange on which a bending load acts, and the ratio of the width to the thickness of the compression-side flange is 12 or less, and the amount of energy absorption is thereby increased. Patent Literature 5 discloses an automotive composite structure member including a steel tube and a reinforcing tube disposed within the steel tube so that the strength can be ensured, the reinforcing tube having an outer shape along the inner wall of the steel tube and having ribs formed therein.
Patent Literature 6 discloses a filling structure in which in order to ensure corrosion resistance, a filling material having a superior energy absorption capability is disposed within a hollow member and is fixed to the hollow member. Patent Literature 7 discloses a car body structure member that is formed of a plurality of members having different strength such that a torsional moment is generated, in order to improve the energy absorption efficiency by distributing a bending load to other members. Patent Literature 8 discloses a bumper structure in which a crush preventing member is disposed within a hollow portion of a bumper reinforcing plate, in order to improve the shock energy absorption capability due to buckling deformation.